Re: [PATCH v2] mm: prohibit the last subpage from reusing the entire large folio

[David Hildenbrand]

On 08.03.24 10:27, Barry Song wrote:
> From: Barry Song <v-songbaohua@xxxxxxxx>
>
> In a Copy-on-Write (CoW) scenario, the last subpage will reuse the entire
> large folio, resulting in the waste of (nr_pages - 1) pages. This wasted
> memory remains allocated until it is either unmapped or memory
> reclamation occurs.
>
> The following small program can serve as evidence of this behavior
>
>   main()
>   {
>   #define SIZE 1024 * 1024 * 1024UL
>           void *p = malloc(SIZE);
>           memset(p, 0x11, SIZE);
>           if (fork() == 0)
>                   _exit(0);
>           memset(p, 0x12, SIZE);
>           printf("done\n");
>           while(1);
>   }
>
> For example, using a 1024KiB mTHP by:
>   echo always > /sys/kernel/mm/transparent_hugepage/hugepages-1024kB/enabled
>
> (1) w/o the patch, it takes 2GiB,
>
> Before running the test program,
>   / # free -m
>                  total        used        free      shared  buff/cache   available
>   Mem:            5754          84        5692           0          17        5669
>   Swap:              0           0           0
>
>   / # /a.out &
>   / # done
>
> After running the test program,
>   / # free -m
>                   total        used        free      shared  buff/cache   available
>   Mem:            5754        2149        3627           0          19        3605
>   Swap:              0           0           0
>
> (2) w/ the patch, it takes 1GiB only,
>
> Before running the test program,
>   / # free -m
>                   total        used        free      shared  buff/cache   available
>   Mem:            5754          89        5687           0          17        5664
>   Swap:              0           0           0
>
>   / # /a.out &
>   / # done
>
> After running the test program,
>   / # free -m
>                  total        used        free      shared  buff/cache   available
>   Mem:            5754        1122        4655           0          17        4632
>   Swap:              0           0           0
>
> This patch migrates the last subpage to a small folio and immediately
> returns the large folio to the system. It benefits both memory availability
> and anti-fragmentation.

It might be controversial optimization, and as Ryan said, there, are 
likely other cases where we'd want to migrate off-of a thp if possible 
earlier.

But I like that it just handles large folios now in a consistent way for 
the time being.

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb